This invention relates to an improved process for impregnating and sealing porous articles such as metal castings using selected water miscible anaerobic sealants wherein excess sealant is easily removed and a cured article is provided with good sealing properties, good heat resistance and low shrinkage.
Porous articles, including metal articles such as castings and sintered metal parts, often need to be impregnated and sealed before use. This is necessary because porosity can cause many different problems, including leakage, and is of particular concern when the articles are subjected to pressure. Impregnating and sealing also improves the articles by strengthening them, reducing their vulnerability to corrosion and preparing their surfaces for subsequent painting or plating operations.
A wide variety of porous metal articles are used commercially today and are manufactured from different metals and alloys such as zinc, copper, iron, aluminum, magnesium and brass. Other materials such as wood, ceramics and fiberglass also frequently need to be sealed.
The need to seal these articles has been recognized by the prior art for many years. The earliest processes generally involved the use of an inorganic sealant such as sodium silicate or a natural organic substance such as varnish. More recently substances such as epoxides, saturated and unsaturated polyesters, and the acrylates have been used in sealing and impregnation techniques.
U.S. Pat. No. 3,672,942 issued Jun. 27, 1972, discloses impregnation with polymerizable anaerobic monomers followed by treating the surface of the impregnated article in an organic solvent solution of an accelerator.
U.S. Pat. No. 3,969,552 issued Jul. 13, 1976 involves an anaerobic impregnation process wherein an aqueous solution of surfactant is used to treat and remove the excess anaerobic sealant from the articles surface.
U.S. Pat. No. 4,165,400 issued Aug. 21, 1979, involves impregnation with anaerobic monomers which are made self-emulsifying and easily removable upon contact with water by incorporating a surfactant in the monomer composition.
U.S. Pat. No. 4,147,821 issued Apr. 3, 1979, discloses a method of impregnating porous articles with a composition of a monofunctional(meth)acrylic ester and a small proportion of polyfunctional (meth)acrylic ester along with a free radical catalyst and inhibitor to effect curing by heating. Excess impregnant is removed by washing with water, if a surfactant is present in the impregnant, or with water containing a surfactant.
In the above noted processes, anaerobic monomers are used with the excess impregnant remaining on the surface of the article being removed by using an organic solvent in one case, by water washing with an aqueous solution containing a surfactant or by water washing an article impregnated with a self-emulsifying composition that already contains a surfactant. Use of solvents involves economic as well as toxicological and ecological disadvantages while the use of surfactants or emulsifiers can have a detrimental effect on the properties and nature of the sealed article. Inclusion of surfactant in the monomer mixture can allow undesired water to be entrapped in the formed polymer. This may result in incomplete sealing since beads of polymer can be separated in the pores by small amounts of water ("popcorn" effect). such entrapped water can also turn into steam at elevated temperatures resulting in a pressure buildup that can destroy the seal.
It has now been found that the use of selected anaerobic water miscible acrylate and methacrylate monomers results in an improved impregnation process where the excess impregnant can be easily removed without the use of an organic solvent or without the use of surfactants either in an aqueous wash solution or in the impregnating composition to render it self-emulsifying and readily water washable. Additionally and significantly, the use of the selected composition provides a sealed article with good heat resistance and good sealing due to low shrinkage of the cured composition. The sealing and impregnation process may be carried out in a system that does not require additives such as plasticizers or thickeners, or a catalyst such as the combination of a tertiary amine and cupric salt, or an inhibitor, to satisfy reactivity conditions. This helps to provide a safer operation, particularly when the vacuum cycle is followed by a pressure cycle, and also maintains good properties in the sealed article that often are deleteriously affected with the incorporation of additives. In addition, the stability of the system is such that refrigeration, frequently required in catalyzed systems, is not needed. Furthermore, the presence of a catalyst system requires constant monitoring and may need the addition of inhibition systems to control sealant stability.